The present invention relates generally to methods for continuously casting steel and more particularly to such methods wherein the steel contains at least one alloying ingredient.
In a method for producing continuously cast steel, molten steel is introduced from a ladle into an intermediate container known as a tundish and then flowed through tundish exit nozzles into a continuous casting mold. Alloying ingredients such as lead and bismuth are added to the molten steel entering the tundish. Lead and bismuth have a relatively low solubility in molten steel and are heavier than molten steel.
Not all the alloying ingredient added to the molten steel is recovered. Accordingly, more alloying ingredient must be added to the molten steel than that which is desired in the final solidified steel, to reflect the fact that the recovery of these elements in the molten steel is less than 100%.
It is desirable that the concentration (Ca) of the alloying ingredient be substantially uniform throughout the cast. Permissibly, a variation in concentration (Ca) should be no greater than .+-.9%, and preferably no greater than .+-.5%. Maintaining such a uniformity in continuously cast steel is particularly desirable where the alloying ingredient has a relatively low solubility in molten steel and has a density greater than molten steel, as in the case of lead and bismuth, but such a uniformity is also desirable for any alloying ingredient added to continuously cast steel.
There are different stages and conditions occurring in the tundish during the casting operation. For example, the step of introducing the steel into the tundish comprises an initial stage, during which the tundish is being filled from a relatively empty condition, and a subsequent stage during which the tundish is relatively full. The mass flow rate (Fs) of the molten steel during the initial stage is greater than the mass flow rate (Fs) of the molten steel during the subsequent stage because it is desirable to fill the tundish as quickly as possible, and so the ladle nozzle, through which molten steel is flowed from the ladle to the tundish, is typically opened all the way during the initial stage of the steel-introducing step.
There are other process conditions or parameters involved in the continuous casting operation, and these include the temperature of the molten steel, the mixing energy to which the molten steel is subjected in the tundish, the residence time of the molten steel within the tundish, and the like.